The present invention relates generally to multichannel receivers and is specifically directed to a signal converter particularly adapted for use in a large bandwidth cable television (CATV) system.
The number of channels CATV utilizes has greatly increased in the last few years and the frequency bandwidth for CATV has expanded from 300 MHz (35 channel capacity) to 440 MHz (60 channel capacity), 550 MHz (78 channel capacity) and an expansion even to 650 MHz (94 channel capacity) is contemplated. These extra channels afforded the benefit of greatly increased programming variety and generated greater revenue for the system. However, this expanded system bandwidth increased the number of interference causing channels significantly and increased also the modulation distortions correspondingly in the signal receiver and degraded the signal reception.
As the number of CATV channels kept increasing, the predominant modulation distortion has been changed from the cross-modulation to composite third order inter-modulation and then to the composite second order inter-modulations.
These modulation distortions are produced by non-linear circuit elements which either transfer the modulation of the non-receiving channel carriers to the desired signal carrier or develop spurious beat products that stack onto the desired signal carrier in a communication receiver when the broadcast signals are evenly spaced which are characterized as the composite triple beats (CTB) or develop spurious beat products, called the composite second order beats (CSB), that stack near the desired signal carrier.
When the frequency bandwidth of the CATV system increased over 500 MHz, the predominant modulation distortion was found to be the composite second order inter-modulation. The second order beats formed by the summation of offending carrier frequencies fall 1.25 MHz above a victim video carrier frequency in the CATV system as shown in FIG. 1A. For the victim Channel 78 carrier frequency of 547.25 MHz as an example, one combination of the two offending carrier frequencies is 55.25 MHz (Ch 2) and 493.25 MHz (Ch 78) and other combinations include any two channel carrier frequencies which add to 548.5 MHz and there are 33 combinations to produce such second order beats for Channel 78. As is clearly shown in FIG. 1A, always one of two such offending channel carrier frequencies are located at lower half of the signal frequency spectrum and a highpass filter with a rejection band attenuation for the lower half spectrum of the signal frequency band will drastically reduce this type of CSB.
The second order beats formed by the difference of two offending carrier frequencies fall 0.75 MHz above a victim video carrier frequency as shown in FIG. 1B for Ch 6 in CATV system. For the victim Ch 6 carrier frequency 83.25 MHz as an example, one combination of the two offending carrier frequencies is 547.25 MHz (Ch 78) and 463.25 MHz (Ch 64) and other combinations include any two channel carrier frequencies which differ by 84 MHz and there are 69 combinations to produce such second order beats for Channel 6. As is clearly shown in FIG. 1B, at least half of two such offending carrier frequencies are located in the upper half of the signal frequency band spectrum and such CSB can be drastically reduced if a low-pass filter with a rejection band attenuation at least for the lower half of the signal frequency band. However, when the signal frequency bandwidth if substantially increased, a simple lowpass or highpass switched filter is inadequate to reduce these composite second order beats sufficiently since in such case the rejection band it required to cover more than half of the signal frequency spectrum and with sufficient rejection attenuation.
Prior art attempts to reduce these distortions have included lowpass-highpass electronic switch such as shown in FIG. 2 and FIG. 3 and also in U.S. Pat. Nos. 4,247,953 to Shinegawa et al, 4,480,338 to Pierr Dobrovolny and the tunable band reject filter extensively used in double conversion type CATV converter shown in FIG. 4. And other include developing more linear receiver components and offsetting channel carrier frequencies to place CSB beats where it is less sensitive to a victim channel. However, these attempts had a limited improvements or were not possible where the receiver is designed for an existing broadcasting medium.
The lowpass-highpass electronically switched filters shown in FIGS. 2 and 3 both essentially possess a fixed pass band and rejection band bandwidth while the circuit in FIG. 3 provides somewhat improved rejection attenuation for the lowpass filter operation. For these filters, the rejection bandwidth is less than half of the signal frequency band and its overall rejection attenuation also is limited due to a rather slow roll-off near the cut-off frequency.
The tunable band rejection filter shown in FIG. 4 is extensisvely used in the CATV double conversion tuner which is tuned by one or two variable capacitance diodes in the filter. These variable capacitance diodes in the filter are controlled with the bias voltage for the voltage controlled first oscillator. And due to the limited tuning range of the variable capacitance diodes in the filter, the tunable rejection band is limited to a rather narrow band located in the high side of the receiver signal frequncy band. Since the rejection band is always for the high side of the frequency band and the filter operates essentially as a lowpass filter, it provides protection against the composite second order beats only for the channels in the low side of the signal frequency band and does not provide any protection at all for the channels in the high side of the signal frequency band where the composite second order beat distortion is usually most severe.
It is accordingly a basic object of the present invention to provide an improved technique for reducing these modulation distortions especially the composite second order beat distortion in a multichannel communication reciever.
It is further object of the invention to provide an effective filtering circuit for reducing these modulation distortions by operating the filter in a lowpass or highpass mode and also in a variable band rejection mode extending the rejection bandwidth with variable capacitance diodes.
It is yet a further object to provide a filtering circuit with a minimum number of circuit elements to achieve effective filtering as those of Elliptic lowpass or highpass filter and whose rejection band is varied and extended for optimum rejection band attenuations.